A conventional electrochemical gas sensor, which is an electrochemical sensor of this type, is designed with such factors as retention of electrolytic solution and permeation of the electrolytic solution to the electrodes being taken into consideration. Therefore, it requires many parts and is difficult to be made compact.
An example of electrochemical gas sensors of this type is shown in FIG. 6. The electrochemical gas sensor shown in FIG. 6 includes a porous base member 51, which made of fluorine-contained resin or the like and is hydrophobic as well as gas permeable. A detection electrode 52 is formed on the base member 51 by printing or other process. Facing the base member 51 is an electrolytic solution retainer 53 which servers to retain electrolytic solution and cause the electrolytic solution to permeate into the electrodes 52,55,56. A porous member 54 is provided to serve as another base member which is hydrophobic as well as gas permeable. The reference electrode 55 and the counter electrode 56 are provided on the porous member 54 and disposed in contact with the electrolytic solution retainer 53. The detection electrode 52, the reference electrode 55, the counter electrode 56, and the electrolytic solution retainer 53 together form a cell 57.
Japanese Patent Provisional Publication No. 1982-147048 discloses a structure wherein electrolytic solution is caused to permeate into electrodes by feeding the electrolytic solution into an electrolytic solution retainer in a cell portion. The cell portion is contained in a case and located towards one end of the case. A hydrophilic liquid guiding member continuously feeds the electrolytic solution from an electrolytic solution storage, which is located towards the other end of interior of the case, into the electrolytic solution retainer in the cell.
Another conventionally known structure is disclosed in Japanese Patent Provisional Publication No. 1994-58906, which calls for causing electrolytic solution to permeate into an electrolytic solution retainer in a cell by bringing the electrolytic solution retainer into direct contact with the electrolytic solution or by using a tubular liquid guiding member. Yet another conventionally known structure is disclosed in Japanese Patent Provisional Publication No. 1994-242059, which calls for causing electrolytic solution to permeate by using a slight gap between the inner wall of an electrolytic solution storage and a ring, which is a separate body from the inner wall of the electrolytic solution storage.
Aqueous solution of sulfuric acid is normally used as the electrolytic solution. However, as sulfuric acid solution is hygroscopic, the sulfuric acid solution used as the electrolytic solution absorbs moisture in the atmosphere and thereby gradually increases the quantity of the electrolytic solution in the electrolytic solution storage of the case. As a result, the pressure in the electrolytic solution storage increases due to the sulfuric acid solution that has absorbed moisture. The increased pressure in the electrolytic solution storage presents the possibility of leakage of the sulfuric acid solution from the electrolytic solution storage.
An example of means to release the pressure to the outside in order to prevent leakage of the sulfuric acid solution from the electrolytic solution storage is disclosed in Japanese Patent Provisional Publication No. 1982-147048 mentioned above, which calls for providing the bottom of the electrolytic solution storage with a gas drain hole that serves as an air vent and closing off the gas drain hole with a porous filter that is hydrophobic as well as gas permeable. The aforementioned Japanese Patent Provisional Publication No. 1994-58906 describes another example, which calls for providing an absorbent inside the electrolytic solution storage to absorb and retain electrolytic solution, thereby absorbing the increased electrolytic solution and, consequently, absorbing fluctuation in the pressure. The aforementioned Japanese Patent Provisional Publication No. 1994-242059 describes yet another example, which calls for permitting air to enter or exit by means of providing porous polytetrafluoroethylene (PTFE) rods or the like inside the electrolytic solution storage.
As described in, for example, Japanese Patent Provisional Publication No. 1994-300735, an electrochemical gas sensor, which is an electrochemical sensor of the type described above, is typically provided with a case that includes an electrolytic solution storage to store electrolytic solution therein. A detection electrode, a reference electrode, and a counter electrode are disposed in the case. Electrode pins are attached to the case. The electrode pins are a plurality of conductors that pass through the case so as to extend from the detection electrode, the reference electrode, and the counter electrode to the outside.
In order to draw outputs from the detection electrode, the reference electrode, and the counter electrode to the outside, each electrode pin is in contact with each respective one of these electrodes. Each electrode pin has a lead portion formed of a metal selected from the group of metals consisting of titanium (Ti), tantalum (Ta), zirconium (Zr), or alloy that contains at least one of these metals. A ribbon portion which has a narrow strip-like shape and made of platinum (Pt) is welded to the distal end of each lead portion. Each ribbon portion is adapted to come into contact and become electrically connected with each respective one of the electrodes, i.e. the detection electrode, the reference electrode, or the counter electrode.
It is also a known practice to form the lead portions of the electrodes of an alloy containing copper (Cu), Nickel (Ni) or titanium.
In cases where the electrolytic solution is drawn up by the hydrophilic liquid guiding member in the case, the liquid guiding member has to be disposed so as to penetrate into the cell. Should there be a plurality of layers of electrolytic solution retainers inside the cell in a structure that calls for the electrolytic solution retainer(s) to have the ability to come into contact with the electrolytic solution, the layers of the electrolytic solution retainers have to be arranged so that the electrolytic solution is sufficiently fed to all the electrolytic solution retainers. Therefore, either structure requires a large number of parts and a complicated production process and is difficult to be made compact.
The structure that calls for using platinum (Pt) to form the ribbon portions welded to the distal ends of the lead portions of the electrode pins of the electrochemical sensor presents the problem of increased production costs, because though platinum is highly anti-corrosive, it is very expensive. Forming the lead portions of the electrode pins of copper or a copper alloy results in superior external pins which are drawn out of the case. Due to poor anti-corrosion properties, however, it is not easy to ensure sufficient durability of such electrodes.
Forming each electrode of two parts, i.e. a lead portion and a ribbon portion, increases the number of parts and the number of welding steps, resulting in increased production costs. Furthermore, there arises the possibility of contact failure resulting from improper welding or electric corrosion at the portion where the lead portion and the ribbon portion are welded to each other, or insufficient mechanical strength, etc. Forming each electrode of a lead portion and a ribbon portion presents such other problems as formation of a local cell, which would impair the measurement precision, and a complicated structure of the electrode pins, which would make it difficult to ensure sufficient reliability.
In order to solve the above problems, an object of the invention is to provide an electrochemical sensor which is easy to produce and make compact.